We propose to design and build a 100 W, 140 GHz gyrotron amplifier and to apply it in time domain DNP/NMR and EPR experiments. During the last decade the repertoire of time domain magnetic resonance techniques that utilize high frequency microwaves in the 90-300 GHz regime has expanded considerably. Specifically, time domain EPR and DNP/NMR experiments have emerged as important techniques for elucidating the structure, function, and dynamic properties of biological systems. However, at present the full implementation of these techniques at high frequency is limited by the lack of microwave power. This proposal requests funding to develop a high power gyrotron amplifier operating at 140 GHz with a 1 GHz bandwidth and 35 dB of saturated gain. A gyrotron amplifier is particularly attractive because it is capable of extension to higher frequency. A low power (30-60 mW) phase switching and pulse forming network (with four phase and <1 ns switching capability) will be used as the input (driver) for the gyro-amplifier. This gyro-amplifier will enable the full implementation of a recently developed electron-nuclear crosspolarization (CP) experiment and the extension to high frequency of recently developed low frequency (9-18 GHz) pulsed EPR techniques for measuring long range (15-80Angstroms) electron-electron dipolar couplings in spin labeled biological systems. To summarize, the specific aims of the research can be divided into three parts: (1) Development of a 100 W, high frequency (140 GHz) gyrotron amplifier. (2) Implementation of electron-nuclear CP experiments with BDPA/PS and trityl/water glycerol samples. (3) Implementation of multiple quantum and SIFTER pulsed EPR experiments at 140 GHz using biradical spin labeled peptides.